This invention relates to polymeric packaging films and more particularly to polymeric films having properties particularly useful for the stretch wrapping of palletized loads of boxes, bags and the like.
It is well known in the art to use thin polymeric films for stretch wrapping of pallets which films are made of polyvinyl chloride, polyethylene and ethylene-vinyl acetate copolymers.
The required properties of a good stretch pallet wrap film are as follows:
1. High machine direction ultimate tensile strength. PA1 2. High machine direction ultimate elongation. PA1 3. High modulus of elasticity. PA1 4. High tear resistance in the transverse direction. PA1 5. High puncture resistance PA1 6. Good optics--clarity and gloss. PA1 7. Good layer to layer cohesion. PA1 8. Low stress relaxation with time. PA1 9. High resistance to transverse tear when under machine direction tension. PA1 10. Producible in thin gauges. PA1 11. Low specific gravity and thus high yield in area per pound.
Polyvinyl chloride film is a popular stretch pallet wrap material but is deficient in having low ultimate tensile strength and elongation, poor puncture resistance, and a high specific gravity yielding a low area per pound. Polyvinyl chloride film is particularly poor in stress relaxation in that it loses 70% of its initial tension after 16 hours when stretched to 30% elongation.
Conventional low density polyethylene is less than an ideal candidate for stretch pallet wrapping because of its relatively low machine direction ultimate tensile and elongation, poor puncture resistance, poor resistance to transverse tear under machine direction tension, and low layer to layer tackiness. Also, low density polyethylene generally cannot be produced in thin gauges particularly from resins having reasonable strength and toughness properties.
While ethylene-vinyl acetate copolymers are potential candidates for stretch pallet wrap film materials, those having less than about 8% vinyl acetate content are similar to and have most of the deficiencies of low density polyethylene. Further, those ethylene-vinyl acetate copolymers having a vinyl acetate content of more than about 8% are normally of lower molecular weight and thus lack the physical properties in terms of the ultimate tensile, tear resistance, impact resistance and stress relaxation properties required.
Illustrative prior art patents relating to this art include U.S. Pat. Nos. 3,433,573 and 3,932,563.